Add API endpoint to query build information

This creates the '/build_info' endpoint and uses PBR to
supply build information.

PBR's current release information is very similar to git describe and
is determined according to the following rules:

1) Get version from setup.cfg (VERSION)
2) Find the most recent annotated git tag object (TAG).
3) Count the number of commits since TAG (X)
4) Get the hash of the current commit (YYYYYY)

The release information will be formatted in one of three possible ways:

1) If TAG matches the current commit:
        release: TAG

2) If VERSION is supplied in setup.cfg:
        release: VERSION.devX.gYYYYYY

3) If a version is not provided:
        release: LAST_TAG.X.gYYYYYY

Implements: blueprint heat-build-info
Change-Id: Ibb2f5afda7c5d7c314e5e062d618405d9f4eb318